On Windows systems with a "utf8" locale, creating std::regex objects for some regular expressions gets stuck. This problem is probably related to issues #441 and #810.

My R session is

R version 4.5.0 (2025-04-11 ucrt)
Platform: x86_64-w64-mingw32/x64
Running under: Windows 11 x64 (build 22631)

Matrix products: default
  LAPACK version 3.12.1

locale:
[1] LC_COLLATE=Italian_Italy.utf8  LC_CTYPE=Italian_Italy.utf8    LC_MONETARY=Italian_Italy.utf8 LC_NUMERIC=C                   LC_TIME=Italian_Italy.utf8    

time zone: Europe/Rome
tzcode source: internal

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

loaded via a namespace (and not attached):
[1] compiler_4.5.0 tools_4.5.0

and the snippet

Rcpp::cppFunction(
  code = '
    void test() {
        std::regex simple("[a]");
    }',
  plugins = "cpp11",
  includes = "#include <regex>"
)

test()

does not end. However, after setting LC_ALL to be "C", "Italian_Italy.1252", or "English_United States.1252", the same snippet works as expected. The problem can also be avoided by using the C++ function std::setlocale() and setting the locale in the test() code.

The issue does not appear to be related to the GNU library: the Rtools g++ can compile the test() function, and the executable behaves as expected even when the locale uses UTF-8. My mingw terminal reports "it_IT.UTF-8" which is consistent with the locale of the R session.